Tunnel vision and you
Some classes suffer from tunnel vision far more than others. Melee classes need to be focused on their enemies (and stay right beside them), or they won't be able to put out damage. Casters, likewise, usually have a broader scope of the battlefield but are still primarily trying to kill only one opponent. Healers can be much more understanding of the big picture than others -- but they can also be focused so heavily on health bars moving up and down that they lose it entirely!
The key is to always be looking around, constantly surveying what people are doing. Sometimes, you'll see a healer running away to drink -- this is an optimal time to load up tons of burst on your opponents. Communicate their weakness with your team and quickly formulate a strategy to punish their disadvantage.
This is even more important in battlegrounds. When you're defending a key objective with two other people and you see five enemies arriving, what do you do? Do you automatically stay and fight? Most people do.
There was probably very little decision-making with that decision, however. Did that defender take into account the classes of the defenders as well as the classes of the attackers? What about health totals? Mana totals? How about who these people even are (you can check what type of player they are by the scoreboard pretty reliably)?
Everyone has the same tendencies
Think of the last time you were defending (or attacking) an objective. When you enter combat with five other people, who is the one you immediately target and start to kill? The one closest to you. Don't believe me? Look for it the next time you play a battleground, and you will be absolutely amazed how close this is to 100 percent.
This kind of thinking isn't just in WoW; it's in first-person shooters as well. If your enemy sees you and a teammate pop around different corners, he will almost always go for the one closest to him. Not only is it a larger target (due to perspective), but it is more deadly to him as well. It's a protective instinct.
This is why good healers stay far in the back. They also stay around other players, even if those players are just other healers. Not only is there safety in positioning (staying back rather than rushing in), but there is safety in numbers. How often do you see someone barrel into five players by themselves, far in front of the group? It happens, but it's pretty rare. Usually teammates will wait until a large group of other teammates arrives before they make their assault on a large force.
Use this to your advantage! Start looking for the healer who is trying to avoid being hassled. Try rushing into a bunch of opponents to tie them up on time if your team needs it (this is one of my favorite strategies for capturing nearby objectives).
Use your focus
If you haven't been using focus, you've been missing out. Focus is a brilliant built-in user interface option that works kind of like a secondary target. Set focus is a keybind in your keybinds menu that allows you to see what another player is doing (usually a healer) who is involved with the battle but not your kill target.
It's helpful for two reasons: It allows you to see cast bars you normally wouldn't be able to see. You can see who your opponents are healing and when that heal will go off. It also allows you to create macros to interrupt those spells or do funky things with Charge, Fear, and the like.
This, if put in a macro, it will turn your target into a sheep, because without a target modifier, the target is your target [target=target].
/cast [target=focus] Polymorph
After you've set your focus and created the above macro, you can use this macro to Polymorph not your target, but your focus! This allows you to surprise enemy players who expected you to cast that Polymorph on your target instead of your focus (which is hidden information for them).
You can do this with any ability that has a target. You can Charge your focus target, Spell Lock, cast Flash of Light, or anything, really. The sky is the limit.
Future PvP videos
On another note, I got the go-ahead to make some PvP videos for you guys. I'll probably be doing a few rated battlegrounds videos as well as arenas in the upcoming weeks. I downloaded a way to record today and will be tinkering with editing and uploading to WoW Insider's YouTube page. I haven't done this in the past, so don't expect greatness. It might be two weeks until the first one is up, but don't worry!
I'll be starting with my warlock, Ceftobiprole of Auchindoun. He's currently my only 85 (I've been really busy lately, OK? Don't laugh at his gear too hard). I might be leveling up my paladin, warrior, mage, and death knight shortly as well to get them in on some PvP, especially if these videos go over well.
Listening Music: 2 Bit Pie with Colours. Just song -- no video, because I couldn't find one. I like this one a bit more than Here I Come, although that one is pretty sweet, too.
Want to ascend the arena ladders faster than a fireman playing
Donkey Kong? We'll steer you to victory with the best arena addons and let you in on some rank 1 gladiator PvP secrets. If you're looking for the inside line on battlegrounds and world PvP, read The Art of War(craft).